To improve the performance of measuring instruments in an ultrasonic measurement, a transducer with a high resolution has been demanded. A transducer is a device for generating or detecting a surface acoustic wave or a bulk acoustic wave. A measurement transducer is used mainly for measurement of material constants, detection of a defect or flaw in an object medium, measurement of stress, and so on. Generally, a piezoelectric substance using the piezoelectric effect which is a phenomenon that the polarization changes by receiving strain due to a sound wave is used as the transducer. The spatial resolution of a measurement system is inversely proportional to the sonic velocity and proportional to the operating frequency. Thus, in order to perform the above measurement with a high resolution, it is necessary to (i) use a shear wave having a lower sonic velocity than a longitudinal wave and (ii) perform the wave generation and detection within a higher frequency range. Therefore, a high-frequency shear wave transducer is in high demand in the measurement field.
As mobile communication equipments such as a mobile phone are downsized, signal processing devices used in the equipments have also been demanded to reduce their size. Such devices include a SAW (Surface Acoustic Wave) device. In the SAW device, a Rayleigh wave which is a combined wave of a longitudinal wave and a transverse wave propagating on a piezoelectric material film was conventionally used. Since the Rayleigh wave attenuates when it is reflected on an end surface of the piezoelectric material film, it has been conventionally necessary to provide a reflector to prevent the attenuation. On the other hand, a shear-horizontal type SAW (SH-SAW) device has been used in recent years, where the shear-horizontal type SAW device is a SAW device utilizing the shear-horizontal type SAW consisted solely of the shear wave vibrating in parallel to the piezoelectric film. Since the shear-horizontal type SAW is totally reflected on an end surface of the piezoelectric film, the reflector need not be provided in the shear-horizontal type SAW device, as conventional ones. which enables the downsizing.
The above transducer and shear-horizontal type SAW operate in a high frequency range of several hundred MHz to several GHz. In the piezoelectric substance of these devices, the relation ν=V/(2d) holds among the frequency ν (sec−1), sonic velocity V (m/s), and thickness of the piezoelectric body d (m). Given that the sonic velocity of the shear wave propagating through the piezoelectric body is 3000 m/s to 8000 m/s, in order for the devices to operate within such a high frequency range, the thickness d of the piezoelectric body needs to be several μm to several tens of μm. Piezoelectric material that can be made to have such a thickness includes ZnO, Pb(Zr,Ti)O3 (which is abbreviated as PZT), and polyvinylidene fluoride-trifluoroethylene (P(VDF-TrFE)).
To generate a shear wave, the piezoelectric substance must vibrate in the slide (or shear) mode, and the polarization axis must be vertically oriented in relation to the direction of the electric field. Thus, for the thin films made of PZT or P(VDF-TrFE), polarization processing must be performed by applying a strong electric field (exceeding 50 MV/m) in an in-plane direction. However, it is difficult to perform such processing over a region of several mm or more. On the other hand, for a ZnO thin film, such a polarization processing is not necessary, but the shear wave can be generated by aligning the crystalline orientation. For example, when the c-axis is oriented to one direction in the plane of the thin film, a shear wave is generated by placing the thin film between electrodes to make the c-axis perpendicular to the direction of the electric field. Thus, it is desirable that the ZnO thin film oriented to one in-plane direction (hereinafter referred to as “c-axis in-plane oriented ZnO thin film”) is used as the piezoelectric film used in the above transducer and shear-horizontal type SAW.
When the ZnO thin film is epitaxially grown on a sapphire single crystal substrate whose surface is set to be the (01-12) plane, the c-axis can be oriented to one in-plane direction. However, when a shear wave transducer is to be produced using the ZnO thin film, the ZnO thin film must be adhered through an adhesive layer to an electrode formed on a surface of an object medium in which the transverse wave propagates. The existence of the adhesive layer lowers the efficiency of converting the vibration of the ZnO thin film to the shear wave which propagates through the object medium. In addition, the sapphire single crystal substrate is expensive and disadvantageous in terms of costs. Furthermore, since the type of the substrates is limited, the characteristics of the device to which the transducer is applied may be restricted.
Thus, it is studied that the c-axis in-plane oriented ZnO thin film is directly formed on the electrode. Patent Document 1 describes that when a ZnO thin film doped with aluminum or aluminum oxide is formed on an aluminum electrode, its c-axis lies in the plane. However, according to this method, aluminum or aluminum oxide is necessarily contained as impurities in the ZnO thin film. Patent Document 2 describes that a low-resistivity ZnO thin film is epitaxially grown on a sapphire (01-12) single crystal substrate as an electrode, and then a high-resistivity ZnO thin film is grown on the epitaxially-grown film as the piezoelectric body. However, in this method, since the electric resistivity of the electrode (low-resistivity ZnO thin film) is higher than that of metal, it is difficult to be applied to various electric devices.
On the contrary, the present inventors found that the c-axis in-plane oriented ZnO thin film can be obtained by depositing the material of the thin film on the substrate endowed with a temperature gradient (Patent Document 3). According to this method, the c-axis in-plane oriented ZnO thin film can be directly formed on a metal substrate (electrode) without doping impurities. For this reason, the c-axis in-plane oriented ZnO thin film obtained according to this method can be suitably used for devices such as transducers and surface SH wave devices.
Furthermore, according to this method, the c-axis in-plane oriented ZnO thin film can be produced on various substrates such as a glass substrate or a ceramic substrate as well as the metal substrate. In addition, this method can be applied not only to the c-axis in-plane oriented ZnO thin film, but also to a thin film having a predetermined crystal axis oriented to a predetermined direction.
Patent Document 3 uses a magnetron sputtering device for depositing the material (ZnO) of the thin film on a substrate. FIG. 1 shows an example of a thin film producing apparatus using the magnetron sputtering device. A magnetron circuit 12 and a cathode 13 are provided in a lower portion of a film formation chamber 11, and an anode 14 is provided in the upper portion. A substrate 10 is placed on a substrate base 15 just below the an anode 14 so as to be substantially parallel to the cathode 13 and the anode 14. A temperature gradient in the direction parallel to the substrate 10 is formed in the substrate 10 with a heater 16 and a water cooler 17 which are provided at the substrate base 15. An additional temperature gradient is endowed to the substrate 10 because of the temperature gradient naturally formed in the film formation chamber 11 by placing the substrate 10 at a position displaced from the center (the chain line in the drawing) of the magnetron circuit 12, the cathode 13 and the anode 14. A target 18 as the material of the thin film is placed on an upper surface of the cathode 13. The magnetron circuit 12 is placed under the cathode 13. The film formation chamber 11 is connected to a gas source 19 of argon (Ar) gas and oxygen (O2) gas.
Operations of this apparatus are described. The Ar gas and O2 gas are introduced into the film formation chamber 11, and a radio frequency electric power is supplied to the cathode 13. A magnetic field and an electric filed are formed in the film formation chamber 11, and the Ar gas and O2 gas are ionized by the electric field to release electrons. The electrons move along troidal curves in the magneto-electric field near the target 18. Thereby, plasma is generated in the vicinity of the target 18 to sputter the target 18. The sputtered material forms a unidirectional flow (material flow) directed to the anode 14 in the plasma. The material flow reaches the surface of the substrate 10 and the sputtered material is deposited on the surface. At this time, the c-axis of ZnO is oriented in the direction parallel to the substrate due to the above temperature gradient.
[Patent Document 1] Examined Japanese Patent Publication No. S50-23918 (the left column, line 36 of page 1 to the left column, line 2 of page 2)
[Patent Document 2] Unexamined Japanese Patent Publication No. H8-228398 ([0017] to [0025])
[Patent Document 3] Examined Japanese Patent Publication No. 3561745 ([0020] to [0031], and FIG. 3)